Conventionally, layout design of a milliwave or microwave IC using a computer aided design (CAD) system has been carried out as follows: storing circuit element data required for making a desired IC in a data base, determining a margin for alignment on the basis of mask alignment precision and a processing precision in transcribing step of an IC production process as a design rule employing a data format for producing a specific mask pattern (for example, CALMA GDSII format), and producing each mask pattern corresponding to each transcribing step manually on the basis of this design rule. Here, the mask pattern is a pattern corresponding to each transcribing step, which is drawn on a mask or a reticle employed in the IC production process.
However, the mask pattern data is necessary for each of the transcribing steps of the IC production process. As described above, since the circuit designer must produce a mask pattern in each transcribing step considering the design rule in the process, skill is required for inputting and editing the data in producing the mask pattern, and further the mask pattern must be modified whenever the IC production process is changed. Such work requires much effort and a long time.
Then, recently, as a method for reducing data inputting work by manual operation, a method for designing a layout of a monolithic microwave IC (hereinafter referred to as MMIC) by CAD system using a software on the market (for example, Academy by Eesof, MDS of HP, Microwave Musician by Cadence, Serenade by Compact Software), which can automatically generate a mask pattern of each circuit element from a circuit diagram of an IC to be designed, has been examined. FIG. 18 is a flowchart illustrating this method of layout design.
In this method of layout design, as shown in this flowchart, first, a mask pattern of each circuit element of an MMIC to be designed is automatically generated on a cathode ray tube (CRT) display from a circuit diagram of the MMIC, and the automatically generated mask patterns of the respective circuit elements are connected with each other, obtaining a diagram of a mask pattern of the whole MMIC. Then, the mask patterns for connecting parts are input according to the connecting methods between the circuit elements to be connected, and the mask pattern of the whole MMIC is edited by, for example, leaving a space between the mask patterns of the respective circuit elements generated in the respective transcribing steps according to the design rule of the production process. Electrical parameters are extracted from the mask pattern of the whole MMIC after editing, and a circuit simulation is executed. When desired electrical characteristics are obtained, the operation is completed. When desired electrical characteristics are not obtained, by changing the size and the configuration of the mask patterns for the connecting parts and re-editing the mask pattern of the whole MMIC, the layout of the mask pattern of the whole MMIC is optimized.
Here, in the above-described flow, after producing the diagram of the mask pattern of the whole MMIC, the mask patterns for connecting parts are input according to the connecting methods between the respective circuit elements to be connected, because conductor layers having terminals used for connection are different for every circuit element, such as an FET, MIM capacitor, and airbridge of the MMIC, and the connecting methods are different according to the kinds of circuit elements to be connected. After inputting the mask patterns for connecting parts, the electrical parameters are extracted and a circuit simulation is executed, because it is necessary to adjust the electrical parameters of the whole MMIC considering the mask patterns for connecting parts, since the size and the configuration of the connecting parts between the circuit elements influence electrical characteristics such as reflection, loss and phase in the milliwave or microwave IC.
The conventional method for designing the layout of the MMIC includes the above-described steps. Therefore, the data of the automatically generated mask pattern for every circuit element includes a pattern portion based on the design rules of the production process, i.e., electrically meaningless excessive data. In order to extract the electrical parameters and execute a circuit simulation as described above, a memory with a large capacity over 10 MB is required. As a result, not only the design device itself (CAD system) is more expensive but also its processing speed is slower, lengthening the times for optimizing the layout. Especially, when a change in the production process is required, new mask patterns on the basis of the design rule defined by the changed production process must be produced and input, and after editing the mask pattern of the whole MMIC, electrical parameters must be extracted and a circuit simulation must be executed again, further unfavorably lengthening the times for optimizing the layout.
It is required to produce mask patterns of the respective circuit elements for as many as transcribing steps as necessary for forming the respective circuit elements in the IC production process. In addition, when the IC production process is changed as described above, new mask patterns must be formed for every changed transcribing step, resulting in high costs in developing and maintaining the CAD program.
In the conventional CAD system, the terminal of a circuit element is defined by a point, and the circuit elements are electrically connected by points. Therefore, for example, when a center point of a line width of a microstrip line (a mask pattern for a microstrip line) is determined as a connecting point, as shown in FIG. 19(a), two mask patterns for microstrip line 2d and 2e extending in the same direction and having different widths from each other cannot be connected smoothly. And, as shown in FIG. 19(b), in connecting two mask patterns for microstrip lines 2f and 2g, at right angles the mask patterns must overlap each other in the vicinity of the connecting point, or a open area without a conductor is unfavorably formed in the vicinity of the connecting point. When such a discontinuous portion where the mask patterns are not smoothly connected is present, microwaves are reflected by this portion or radiation loss is increased. As a result, the mask patterns must be modified on the CRT display, which is a troublesome operation.
The above-described problems occur in designing the layout of not only a microwave IC but also a milliwave IC.